This invention relates to an improved method of gauging the wall thickness of a tubular object, such as a seamless steel pipe, in a non-contacting manner by the use of radiation.
When a beam of radiation, such as a gamma-ray, passes through a material the intensity of the radiation beam generally decreases with the distance, due to absorption or scattering of the beam in the material. The intensity of the radiation beam may be considered as the number of photons or radiation particles, and more specifically as the number of counts indicated by a radiation detector, and may be expressed as: EQU N=N.sub.0 e.sup.-.mu.x ( 1)
where N denotes the intensity of the radiation beam, N.sub.0 is an initial value of the intensity at a position before the beam enters the material, e is the base of the natural logarithm, .mu. is an absorption coefficient, and x is the length of the transit path of the radiation beam across the material layer. The absorption coefficient .mu. is a value determined by the energy of the gamma ray and by the type of material being measured. For example, if the radiation source is caesium 137 having a gamma ray energy of 0.622 MeV and the material is iron, the coefficient .mu. is approximately 0.06 [1/mm].
More precisely, the above Equation (1) indicates an idealized formula, if the transit path length x is larger, it is modified and expressed as: EQU N=N.sub.0 Be.sup.-.mu.x ( 2)
where B is a regeneration factor. It may be expressed also as: EQU N=N.sub.0 e.sup.-.mu.x, .mu.=.mu.(x)
where .mu. is variable.
A method of gauging the wall thickness of a steel pipe using radiation is known from the 1979 Japanese patent application No. 114263 and is illustrated in FIGS. 1 and 2. The tube 1 the wall thickness of which is to be gauged is presumed to have true cylindrical and coaxial outer and inner peripheral surfaces having respective radii R.sub.1 and R.sub.2. A gamma ray beam 2 is used to scan the tube 1 by moving in the direction lateral to the axis of the tube 1.
The y-axis is set to coincide with the direction of the lateral movement of the gamma ray beam, and the y-coordinate is zero at the position corresponding to the center of the tube 1. The length of the path of the gamma ray beam across the tube wall is denoted as x, and N is the detected intensity of the gamma ray beam after it transits the pipe. The axis of the radiation beam is perpendicular to the y-axis. The value of x is thus expressed as: ##EQU1## The value of N is expressed as: ##EQU2## If the positions of the inflection points S.sub.1 (y=R.sub.1) and S.sub.2 (y=R.sub.2), or S.sub.3 (y=-R.sub.2) and S.sub.4 (y=-R.sub.1) of the curve showing the value of detected radiation beam intensity N can be determined, the examined tube wall thickness H may be expressed as the difference between them in the y-coordinate.
The above known method of gauging the tube wall thickness includes finding a point of minimum attenuation of radiation transmission where the radiation beam tangentially contacts the outer peripheral surface of the tube, and a point of maximum attenuation of radiation transmission where the beam tangentially contacts the inner peripheral surface of the tube. The distance therebetween is the tube wall thickness.
It is a disadvantage of this known method, however, that determining accurate positions of the points S.sub.1 and S.sub.2 or S.sub.3 and S.sub.4 requires a fairly long time. Also, inaccurate results may be obtained because it is not easy to determine the inflection points of the variation of detected radiation intensity during actual measuring operations.
To form sharp inflection points a very high resolution of the radiation beam is needed, which requires a radiation beam narrowed by a collimator assembly into as thin a beam as possible. With reference to FIG. 2, the gamma ray from a source 3 passes through a slit having a thickness .DELTA.y of the first collimator member 5 near the source 3, to form a sector-shaped beam 2a. The slit of the second collimator member 5a near the detector 4 narrows the beam 2a into a thin beam having thickness of .DELTA.y. However, reducing the radiation beam thickness also reduces the radiation energy reaching the detector 4 per unit of time. Accordingly, a fairly long time is required for the measurement operation, during which time the measuring system (i.e. the radiation beam generating device and the detector) must be at a standstill in relation to the tube being examined.
Also, the indication of the detected radiation (except in X-ray measurement) generally is inevitably accompanied by error, referred to as a statistic noise, the value of which is proportional to .sqroot.N, where N denotes indication of detected radiation. That is: ##EQU3##
Consequently, the larger the indication of detected radiation N, the smaller the relative error becomes. It is, therefore, necessary to have the amount of radiation energy reaching the detector greater than a certain minimum value to obtain an accurate measurement. For example, where a tube being examined has a wall thickness of 20 mm and a resolution of 0.1 mm is needed in its measurement, it is necessary to have more than 200 measuring points.
A collimator, as referred to above, includes a massive radiation shield formed, for example, of lead 50 mm or 100 mm thick. Assuming a straight hole is bored through the shield having a diameter of 0.5 mm through which the radiation beam passes (although this may be smaller than the smallest practicable diameter in a lead shield), and assuming the radiation source is caesium 137, the distance between the source and the detector is 600 mm, and the detection efficiency is 50%, then the radiation energy N.sub.0 reaching the detector with no absorption material interposed between the source on the detector is approximately 683 cps (counts per second). To lower the statistic noise below about 1/500, the amount of radiation energy required to reach the detector is more than about 2.5.times.10.sup.5 counts. Consequently, about 6 minutes is spent for one step of the measurement operation at each measuring point. Therefore, a complete process for obtaining a single value of the tube wall thickness comprising 200 measuring points requires about 20 hours.
As described above, the known method is impractical for actual tube wall thickness measurement, particularly in industrial processes for manufacturing long continuous tubular products, such as seamless steel pipes where a quick, on-line thickness measurement is required.
If X-rays are used instead of gamma rays, there is no statistic noise problem. However, X-rays result in a low detection efficiency, so that a relatively long time is required for the measurement operation to determine sharp inflection points.
It is an object of the present invention to eliminate the above disadvantages of the known method by providing a method of tube wall thickness measurement applicable to actual industrial processes for the manufacturing or inspecting of tubular products such as, for example, seamless steel pipes in hot rolling lines or in cold inspection lines, where each tubular product moves past the measuring equipment for a period of time not more than several ten seconds.
It is a further object of the invention that the measurement method be applicable to on-line operations.